It is known to use a vacuum bag molding process to pressurize while heat curing a composite structure. To obtain a quality part it is necessary to remove all the trapped air from a layup being formed into the part, however, the path for removal of air also provides a path for excessive amounts of resin to flow during the cure cycle. In U.S. Pat. No. 3,703,422 a vacuum bag molding process is shown with layers of glass cloth communicating between a vacuum source and bleeder layers covering a panel being formed. The layers of glass cloth not only provide a path for removal of air, but also a path for resin to bleed or flow into when the resin viscosity is lowered during the cure cycle. In another known method a porous parting layer covers the layup and the parting layer in turn is covered by a glass cloth layer. The glass cloth layer acts as a bleeder to remove air, however, during the heating cycle excess resin bleeds into the glass cloth layer. The cure cycle is accomplished in two steps. After the vacuum is applied the unit is first heated up to a temperature where the resin begins to gel and is held at that temperature for a time to partially cure the resin and prevent excessive run off. Next pressure is applied to the outside of the vacuum bag and the temperature raised to and held at the cure temperature for a time sufficient to cure the resins. Even with this controlled rate of resin run off the resin lost into bleeder layers often amounts to 25% or more and varies with different resins and different suppliers' resins. To obtain the desired resin to fiber percentage in the finished product one provides an excess of resin for run off and tries to control the amount of the run off.
It was found that non-bleed vacuum bag molding can be accomplished wherein the air is completely removed from a layup, but the breather means closes off when resin starts to flow into it, and the cure is accomplished in one step instead of two steps.